Study Guide

What is Sand?

Sand is a fascinating material, inspiring people's imaginations and creativity. Sand can be sculpted into an infinite variety of shapes. The fine grains pour easily through one’s fingers, and yet with a little moisture can be packed tightly together. Children and even adults spend countless hours building sand castles on the beach. Sand boxes in playgrounds are a major attraction. But what is sand exactly?

Well, here are the nitty-gritty details! Sand is made up of minerals and tiny pieces of rock that were once part of another, much larger rock. Over time, the process of weathering can turn large rocks into sand, and erosion then carries the pieces away.

The most common ingredient in sand is quartz. But the composition of sand will vary from place to place, depending on the makeup of the local rocks. Geologists can often track down the original source for sand grains by comparing the minerals and other characteristics of the sand with rocks in nearby mountains.

From a distance, dunes appear to be made of all similarly sized and colored grains of sand. However, upon closer inspection one can see a mixture of assorted grain sizes, shapes, and colors. Various sections of the same dune may range from fine-grained to coarse-grained sands. Even wind ripples can concentrate sand grains of particular sizes on a smaller scale.

In general, particles from 0.06 mm to 2.0 mm in diameter are classified as sand. After that, sand is commonly divided into five sub-categories: very fine (1/16 - 1/8 mm), fine (1/8 mm - 1/4 mm), medium (1/4 mm - 1/2 mm), coarse (1/2 mm - 1 mm), and very coarse (1 mm - 2 mm). The farther the material has traveled, the smaller and more rounded the sand particles will be, as a result of buffeting action against the ground and other sand grains.

Below are some sand samples from different southwestern dunes. As you look at each photo, note the grain size and color as well as the degree of rounding and sorting of the grains. Does the sand seem to be mainly composed of one type of mineral or several kinds?

NOTE: Top photo is ordinary table salt for the purpose of a size comparison.